1. Field of the Invention
This invention was made with Government support and the Government has certain rights in this invention. This invention relates to fullerenes and to a process for making macroscopic amounts of fullerenes.
2. Description of the Prior Art
A fullerene is a third form of pure carbon and is different from graphite and diamond, the only two forms known before 1985, see R. F. Curl and R. E. Smalley, "Fullerenes," Scientific American, October, 1991, pp. 54-63, incorporated herein by reference, and references cited therein. A fullerene structure is generally characterized as having each carbon atom bonded to three other carbon atoms. The carbon atoms curve around to form a molecule with cage-like structure and aromatic properties. One fullerene molecule referred to as "buckminsterfullerene" contains 60 carbon atoms bonded together in a spherical relationship, a structural diagram of which resembles the familiar shape of a soccer ball.
The molecular structure for buckminsterfullerene was first identified in 1985, see Kroto, et al., "C.sub.60 : Buckminsterfullerene", Nature, Vol. 318, No. 6042, pp. 162-163, Nov. 14, 1985. The process for making fullerenes described therein involves aiming a focused pulsed laser at a rotating solid disk of graphite to vaporize the carbon. The carbon vapor was then carried away by a high-density helium flow. That process did not utilize a temperature controlled zone for the growth and annealing of fullerene molecules from the carbon vapor formed by the laser blast, and only microscopic quantities of fullerenes were produced. Shortly thereafter, the laser technique was adapted to produce microscopic quantities of fullerenes containing a single atom of lanthanum inside the fullerene structure, see Heath, et al., "Lanthanum Complexes of Spheroidal Carbon Shells," J. Am. Chem. Soc., vol. 107, pp. 7779-7780, (1985).
The fullerene yields from laser vaporization were improved by providing a temperature controlled space for the carbon atoms in the carbon vapor to combine in a fullerene structure, see, Chai, et al., "Fullerenes with Metals Inside," J. Phys. Chem., Vol. 95, No. 20, pp. 7564-7568 (1991). This process was adapted to produce macroscopic amounts of fullerenes having one or more metal atoms inside, see Haufler, et al., "Carbon Arc Generation of C.sub.60," Mat. Res. Soc. Symp. Proc., vol. 206, pp. 627-637, (1990), and Chai, et al., "Fullerenes With Metals Inside," supra.
Another method of making fullerenes was described by Ajie et al., "Characterization of the Soluble All-Carbon Molecules C.sub.60 and C.sub.70," J. Phys. Chem. Vol. 94, No. 24, 1990, pp. 8630-8633. The fullerenes are described as being formed when a carbon rod is evaporated by resistive heating under a partial helium atmosphere. The resistive heating of the carbon rod is said to cause the rod to emit a faint gray-white plume. Soot-like material comprising fullerenes is said to collect on glass shields that surround the carbon rod.
Another method of forming fullerenes in greater amounts is described by Haufler, et al., "Efficient Production of C.sub.60 (Buckminsterfullerene), C.sub.60 H.sub.36 And The Solvated Buckide Ion," J. Phys. Chem., Vol. 94, No. 24, pp. 8634-8636 (1990).
One disadvantage of the prior art is the low yield of fullerenes. Another disadvantage of the prior art is the relative difficulty with which large amounts of carbon are vaporized continuously and then condensed into soot comprising fullerenes. Although the prior art methods described above are suitable for production of fullerenes at relatively low rates, e.g. 10 grams per hour of (@C.sub.60) and other fullerenes, they have not been efficiently scaled up to produce a soot comprising fullerenes at high rates, and having a high yield of fullerenes.
An intrinsic difficulty with vaporizing carbon to produce fullerenes is that the carbon source must be heated to over 2800.degree. C. At these temperatures, black body emission is intense. The intense light produced by black body emission diminishes the yields of fullerenes in prior art processes, because large carbon clusters form in the carbon vapor while the carbon vapor has a relatively high carbon concentration and is relatively close to the intense light.
Although fullerenes such as (@C.sub.60) absorb all wavelengths below about 7,000 angstroms, absorption of ultraviolet (UV) radiation (wavelengths below 3,500 angstroms) is 10-100 times stronger than light in the visible wavelengths. In prior methods of vaporizing carbon utilizing an electric arc, the plasma at the core of the arc can have a temperature in excess of 10,000.degree. C. and will produce a large amount of UV radiation. The intense UV radiation from the plasma promotes photochemical destruction of fullerenes. This is because newly formed fullerenes moving away from the region very close to the arc are exposed to intense UV radiation and are excited to a triplet state for a few microseconds after absorption of the UV radiation, see Haufler, et al., Chem. Phys. Lett., 179, 448 (1991).
While a fullerene is in an excited triplet state, it is an open-shell species, that is more susceptible to reaction with other carbon species than in its normal closed-shell state. The result of the reactions of the carbon radicals in the excited triplet state is carbon clusters having very large numbers of carbon atoms. The reaction products are insoluble in the solvents normally used to dissolve smaller fullerenes.
The rate of the photochemical reaction of the carbon species in the excited triplet state increases linearly with photon flux and carbon radical concentration. In the prior art method of vaporizing carbon with an electric arc, the photon flux, especially UV radiation, and the carbon radical concentration are both relatively high within a few diameters of the carbon electrode tips. As the diameter of the carbon electrode is increased, the region where the photochemical reaction takes place and the high concentration of carbon radicals exists becomes larger in direct proportion to the electrode diameter. With carbon electrodes of relatively small diameters, the fullerenes are exposed to extremely intense UV radiation only briefly, and if carbon rods are heated resistively, only small amounts of UV radiation are produced. However, larger diameter electrodes, that would be used to make fullerenes at high rates, produce large amounts of UV radiation which causes a photochemical reaction of the newly formed fullerenes with the carbon vapors.